1. Field of the Invention
The present invention relates to an optical disc recording/reproducing device, and more particularly, to an optical disc recording/reproducing device and a method thereof for promptly re-focusing the focal point after the first focusing was completed and the focal point is defocused again.
2. Description of the Related Art
FIGS. 1A and 1B schematically show the relationship of the relative position between the focal point of the optical pick-up head and the data layer in the optical disc. Referring to FIG. 1A, the conventional optical disc recording/reproducing device needs to focus the light beam generated by the optical pick-up head onto the data layer 111 of the optical disc 110 (e.g. the focal point 113b shown in the diagram) before performing the subsequent data reading (also known as reproduce) operation. However, during the data reading, the focal point 113b, which had been focused on the data layer 111, may depart from its accessible range due to various factors such as defects on the optical disc 110, deviation on the spinning axis direction or the external force interference. Meanwhile, the optical pick-up head has to perform the focusing operation again, that is to pull down the focal point generated by the optical pick-up head below the data layer 111 (e.g. pull down the focusing point generated by the optical pick-up head to the focal point 113a) and then to push up the focal point generated by the optical pick-up head. When the focal point approaches the data layer 111 and has entered into a linear control area of the error focusing, the optical disc recording/reproducing device immediately enters into a closed-loop control loop for accomplishing the focusing operation. Wherein, the details of the linear control area of the error focusing and the closed-loop control loop are described in greater detail hereinafter.
However, since the degree of deviation of the optical disc 110 on the spinning axis direction is uncertain, the conventional method directly pulls the optical pick-up head down to a lowest position allowable by the mechanism of the optical pick-up head, and the optical pick-up head up is pushed up again to perform the focusing operation. In general, in a case where the data layer 111 of the optical disc 110 is already on the lowest position, the lowest position allowable by such mechanism is sufficient for the optical pick-up head to perform the focusing operation. In addition, the data layer 111 is usually on the outer edge of the optical disc 110 when it is on the lowest position as mentioned above. Referring to FIG. 1B, when the optical disc 110 is deviating on the spinning axis direction, the reference surface of the data layer 111 on the outer edge 115a is lower than the reference surface on the inner edge 115b, wherein the reference surface is perpendicular to the spinning axis. Use the inner edge 115b of the data layer 111 as an example. When the focal point departs from the accessible range and the optical pick-up head needs to be re-focused, the conventional technique first pulls the focal point generated by the optical pick-up head down to the lowest position allowable by the mechanism (e.g. the position 117 shown in the diagram) and then pulls the focal point generated by the optical pick-up head up to the position of the inner edge 115b. Accordingly, every time when the focal point is departed from the accessible range, the conventional focus-searching mechanism takes a certain period of time, resulting a great part of the time is wasted from the data reading is interrupted until it resumes.